1. Field of the Invention
This invention relates to a rotary damper for use with a rotating or sliding member such as the cover of a cassette tape recorder, a computer floppy disk drive, a glove compartment or an ash tray in a car or other such device, and which damps rotating or sliding movement of the member making use of the viscosity of grease or silicone oil.
2. Description of the Prior Art
U.S. Pat. No. 4,701,229 discloses a rotary damper wherein a dish-shaped housing has a circular depression, viscous fluid is charged into the depression, a damping rotor with a revolving shaft standing from the center thereof is horizontally accommodated in the viscous fluid in a rotatable manner, and a cap having a central hole for passage of the revolving shaft of the aforesaid damping rotor therethrough and a cylindrical wall of the dish-shaped housing surrounding the aforesaid depression are fixed to each other by welding so that the depression is covered by the cap from the upper side.
In the aforesaid rotary damper, a downward inner circular protruding wall is provided along the external periphery of the cap, while an upward outer circular protruding wall is provided along the internal periphery of the cylindrical wall of the dish-shaped housing, the external peripheral surface of the inner circular protruding wall and the internal peripheral surface of the outer circular protruding wall are opposed to each other, and welding is effected by pressing a high-frequency welder horn onto the two circular protruding walls to melt upper edge sections of the two circular protruding walls by high-frequency vibration.
The engagement inevitably leaves a small clearance between the external peripheral surface of the cap including the aforesaid inner circular protruding wall and the internal peripheral surface of the cylindrical wall including the outer circular protruding wall, and a lower edge of this small clearance communicates with the inside of the depression of the dish-shaped housing filled with viscous fluid. For this reason, the viscous fluid is drawn into the thin clearance by capillary attraction. As a result, the section to be welded may sometimes be wetted by the viscous fluid. If this happens, the weld may be weak or irregular in shape so that the cap may come off from the dish-shaped housing or the viscous fluid may leak to outside when the fluid gets hot and its volume expands.
An object of this invention is to provide a rotary damper which will not come apart owing to inadequate strength of the weld between the cap and the housing, and which will not suffer viscous fluid leakage.